Many modern sensing and communication devices, such as radar, sonar, and lidar in the case of sensors, and phase modulation communications, make extensive use of phase comparisons of variable-frequency signals to determine information content. One common variable-frequency signal is the linear-frequency-modulation (linear FM) signal, widely used in radar. In general, the amount of information which can be extracted from such devices depends upon the linearity of the linear FM signal. Many of these devices include nonlinear elements, such as frequency converters (upconverters or downconverters) for frequency translation, frequency multipliers for bandwidth expansion, or detectors. The nonlinear devices often adversely affect the phase, which is to say that the phase relationships of the signals entering the nonlinear device are different from those of the signals exiting the device. Moreover, the phase of signals exiting the nonlinear device at a given output frequency may be different from that of signals exiting at other frequencies. That is, the phase error introduced by a nonlinear device is not necessarily a constant, but may instead be a function of the frequency.
It is very desirable when designing systems using nonlinear devices to be able to determine the relative quality of different nonlinear devices which perform the same function, so as to be able to select for use that one device, or those devices, which least perturb the phase, and maximize the linearity of the linear FM. This selection allows the system being designed to extract the maximum possible information from the signals. Thus, phase measurements are made to determine the relative quality of each potential design of nonlinear device.
Prior-art methods for making phase measurements include, for example, applying frequency-swept or linear-frequency-modulated (linear FM) input signal to a nonlinear device, for producing frequency upconverted, frequency multiplied, or detected signals. The output signal from the nonlinear device will, in general, be different from the input signal. In order to compare the phase of the output signal to the input signal, the frequency upconverted or multiplied signal is downconverted back to the original frequency, as by use of a downconverter using the same local oscillator as the upconverter. The phases of the input signal and the downconverted signal can then be directly compared in phase. Thus, this prior-art method requires the use of a second nonlinear device in addition to the upconverter, namely the downconverter. If a downconverter were to be the nonlinear device being tested, the upconverter would be the extra nonlinear device. Unfortunately, this technique only provides an indication of the phase error of a cascade of nonlinear elements, and not of one element alone. While one may make the assumption that the nonlinearity is evenly divided between the upconverter and the downconverter, this is merely an assumption, and it can be very difficult to determine the actual phase errors or contributions of each of the upconverter and downconverter separately. Network analyzer devices currently on the market use this upconversion/downconversion technique, with the disadvantage that the calibration characteristics of the additional nonlinear device may not be well established.
In those cases in which the nonlinear device performs frequency expansion or contraction, as would be the case of frequency multipliers, there is no effective or standard prior art method for determining the phase characteristics of the nonlinear device.
Improved or alternative phase evaluation methods and apparatus are desired for use with nonlinear elements or devices.